square bracket notation is that we can also use variables to get properties dynamically. code example

Example 1: javascript dynamically access object property

var person ={"first_name":"Billy","last_name":"Riley"};
var property_name="first_name";
alert(person[property_name]); //Dynamically access object property with bracket notation

Example 2: dynamic properties instead javascript

var foo = { pName1 : 1, pName2 : [1, {foo : bar }, 3] , ...}

var name = "pName"
var num  = 1;

foo[name + num]; // 1

// -- 

var a = 2;
var b = 1;
var c = "foo";

foo[name + a][b][c]; // bar